Three-dimensional images and pictures have been increasingly used for various purposes and grown in importance for products using the three-dimensional images and pictures such as a three-dimensional display TV, and in order to acquire three-dimensional images for distant military targets, three-dimensional images for monitoring a natural environment such as a landslide, and various three-dimensional environments around a vehicle required for travelling of a unmanned automatic driving vehicle.
In the related art, even inferior-quality three-dimensional images served to perform their roles in some regions. However, in recent years, as application ranges of the three-dimensional images are extended, acquiring superior-quality three-dimensional images even under various environments has become important.
A method for acquiring the three-dimensional images may be implemented by various methods, and not a method of a reflection wave of a modulated optical signal but a method capable of acquiring the three-dimensional images by measuring a time of flight of the reflection wave of light is generally used. The method is characterized in that a measurable range can be set to be very wide and pulse recognition is simple. As another method, there is a method of acquiring reflection information by using a difference in characteristics between a modulated and returned signal and a signal outputted at that time when transmitting an optical signal. This method is more complicated in a system structure than the method using the measurement of the time of flight of the reflection wave of light and is limited in a measurement distance depending on a modulation characteristic, but can have a higher SNR characteristic, such that this method is partially used.
Further, there are various three-dimensional image acquiring methods, but a common problem is that it is not easy to acquire the dynamic range of the image. In the image, when the dynamic range, that is, an optical intensity difference between the brightest portion and the darkest portion is not maximized, the bright portion is too bright, such that it is difficult to distinguish an object and the dark portion is too dark, such that it is difficult to distinguish the object. In the case of the three-dimensional image, in a region without a sufficient dynamic range, not only simple image loss or SNR reduction of a corresponding part occur, but the form of the three-dimensional image to be acquired is lost.
In particular, the three-dimensional image is required in not only a dark portion but also a portion having an optical signal of a bright background such as the sun and optical reflectance of a reflector may also vary very variously depending on a type of a target object and a reflection angle, and as a result, a high dynamic range reception structure can be a very important factor under the above environment.
In the case of an existing general image, the image can be acquired under various exposure conditions by adjusting an integration time corresponding to a light receiving time to collect the optical signal or an aperture opening time in addition to improving a characteristic of an element in order to increase the dynamic range, and as a result, a high dynamic image can be ensured.
That is, regardless of a light detection time, a high dynamic range image can be acquired from an optical signal inputted into a pixel with relatively the same intensity while changing various conditions (the aperture opening time, and a driving time and a driving method of a detection device) by using the detection device having an excellent photo-sensing ability.
However, in the case of the three-dimensional image, since the reflection wave needs to be detected by using a very short optical signal, that is, an optical signal having a pulse width of 1 to 10 nsec in order to acquire high distance resolution and the detection time also needs to be as short as the time, a problem cannot be solved by increasing the integration time used in the existing general image or changing the driving time of the device.
Accordingly, when it is impossible to increase the dynamic range by merely changing the integration time, a method for increasing the dynamic range for the three-dimensional image or picture is required. In the related art, there is a method in which an automatic gain control (AGC) circuit is placed to control a gain of an amplifier such as a trans-impedance amplifier (TIA) in order to prevent signal saturation generated at the rear end of an optical detector such as an avalanche photodiode (APD).